Is space an entity, a relationship or a concetual framework?

Hi, the first paragraph in the Wikipedia entry to space contains a sentence saying there is disagreement between PHILOSOPHERS whether space is an entity, a relationship or a conceptual Framework. I am interested in what PHYSICISTS think about this.

My personal view is that it is an entity as it is something that has physical PROPERTIES. It therefore can and should be investigated by physicists. I think the business of physics is the investigation of changes to matter and energy and as space has an energy momentum Tensor, it therefore has properties and should be considered as an entity.
Id like to make a small unrepresntative survey on your view so could I ask anyone reading this to let me know what their view is by just making a small comment like

"Entity" ,
"relationship" or
"Framework".

If you could also leave an indication of what you have studied, it woud help me like -

p for trained physicist,
e for engineer,
m for mathmatician, and
o for other)

Of course if you want to give arguments for one view or the other, it would also be most welcome.Thanks

I think that the consensus among physicists is that space-time is a framework in which things happen. As has been said, time is what keeps everything from happening all at once and space is what keeps it from all happening to me.

The pop-science concepts of "expanding" space, "fabric" of space, ""bending" space are not taken seriously as they are an attempt to render in English things that really only render in math.

From my experience with physicists, not very many worry very much about this sort of question. Many are even pretty hostile to the idea of spending time thinking about this kind of question.

As far as most physicists are concerned (from my experience), space-time is a mathematical structure which is a component of various theories that describe how the universe works. They usually aren't worried with questions like, "Is this an entity or a framework?" but instead, "Does this mathematical structure provide predictions which can be tested experimentally?" They tend to get really frustrated with philosophical questions like the above because they don't see that there's any way to answer them definitively. But when there's an experiment you can do, then it becomes really easy to come to a solution.

There are physicists who are much more friendly to philosophical questions like this, but they seem to be pretty rare.

Exactly what I expected. But here is the thing. I think that at the minimum there is the need for a new vocabulary or nomenclature.

If physicists use words like expanding, contracting etc. it leads the "public" to think of "movement" or "velocity" of space. Here are some suggestions: You can take them seriously, or with a pinch of humour.

Space is "hubbling" - Hubbling refers to the expansion of the framework that is called space.

A recession velocity of a galaxy that is moving faster than the speed of light should be called a " Quasi- velocity" to differentiate between what is a velocity (with a maximum velocity of the speed of light) and the quasi-velocity which is greater than the speed of light.

I think expanding is a pretty good description. There are lots of analogies available to explain the expansion. Adding special new terminology just complicated matters because now that new terminology needs to be explained.

I think the simplest explanation is that the expansion means that galaxies tend to move away from one another, and the further they are, the faster they move.

No. Your ideas are not wrong or even wrong-headed, it's just that they are impractical to the point of being useless. We've had a lot of discussions here about changing the terminology in physics because much of it is so badly representative of what's really going on. The problem is that any "decision" we make here on PF is utterly meaningless in the wider world and it's just a waste of time to worry about it.

Perhaps terminology is the wrong word. Perhaps the word should be concept

Would you say the use of velocity of a body as used in special relativity and the use of velocity of a body when used in reference to a galaxy beyond the cosmic event horizon are the same animals?

Is velocity always the same thing in physics or not? Is it always used the same way and always follows the same rules?

I think that where there are differences, these are different concepts and should also have different terminology

Velocity behaves a bit differently depending upon whether you're talking about Newtonian physics, special relativity, general relativity, or quantum mechanics.

In Newtonian physics, velocity is very simple and intuitive: it's just the rate of change of the object's position.

Special relativity throws a wrench into the mix: every observer measures the speed of light to be c. This complicates math a bit, but it's conceptually not really different from the Newtonian case, except that now there's a speed limit.

General relativity really mucks things up. Locally, General Relativity works exactly like special relativity (so that velocity relationships for objects passing one another are the same). But for far-away objects, relative velocity becomes poorly-defined. Because space-time is curved in General Relativity, you can't directly compare the velocities of two objects: Imagine, as an example, two cars on exact opposite sides of the Earth on the equator that are both moving directly north at the same speed compared to the Earth. How fast is one car moving with respect to the other? If we compare the distance between the cars along the south, they're moving away. If we compare their distance along the north, they're getting closer. If we compare their distance east/west, they're not moving with respect to one another at all. You can come up with a convention to say what the velocity of each car is, but somebody else could just come up with a different convention. There's no way to say who is right. This is why far-away galaxies don't obey the speed of light limit when considering their velocity with respect to us: that speed limit can only apply at a single point, so that in GR the speed of light limit reduces to, "Nothing can outrun a light beam."

Quantum mechanics creates a separate problem because an object no longer has a single speed: as objects in QM are waves, they have a distribution of speeds, with some parts of the wave moving faster than others.

A recession velocity of a galaxy that is moving faster than the speed of light should be called a " Quasi- velocity" to differentiate between what is a velocity (with a maximum velocity of the speed of light) and the quasi-velocity which is greater than the speed of light.

First, you don't want a special word or a special phrase for recession velocities over c, because that'd suggest that there's some physics that changes once a galaxy recedes over that speed. You want a phrase to differentiate regular (local, special relativistic) velocity from any velocity associated with expansion (a general relativistic effect), as this does delineate different physics (Chalnoth explained the differences above). But we already have a phrase for that in cosmology - it's 'recession velocity'. I see no reason to change one noun modifier to another.

From my experience with physicists, not very many worry very much about this sort of question. Many are even pretty hostile to the idea of spending time thinking about this kind of question.

As far as most physicists are concerned (from my experience), space-time is a mathematical structure which is a component of various theories that describe how the universe works. They usually aren't worried with questions like, "Is this an entity or a framework?" but instead, "Does this mathematical structure provide predictions which can be tested experimentally?" They tend to get really frustrated with philosophical questions like the above because they don't see that there's any way to answer them definitively. But when there's an experiment you can do, then it becomes really easy to come to a solution.

There are physicists who are much more friendly to philosophical questions like this, but they seem to be pretty rare.

The problem is it stops being just "philosophy" when statements such as "galaxies move away from each other because space expands" or "light is redshifted because of the stretching of space" are given unscrupulously everywhere. If space is a mathematical framework then it is false to state that the universe works the way it does because of how this mathematical framework behaves, this is the fallacy of reification. They get frustrated with philosophical questions, but they wouldn't have to if they used the proper terminology in the first place.

Why do galaxies move away from each other at a velocity proportional to the distance between them? If you consider space to be a mathematical concept and say because space expands or stretches between them, it's wrong. If you consider space to be an entity, then it is still wrong because this entity hasn't been detected. The valid answer to this question would be "we don't know", or if the big bang is assumed "because that's what they have been doing since the big bang", which then begs the question why that is the case. Explaining why in term of a mathematical concept gives the illusion of an explanation, at best it is an analogy.

In the end it is the non-physicists eager to understand who get frustrated with the misleading explanations given by the physicists. The honest attitude from the physicists then would be to clarify the misconceptions they are responsible for spreading in the first place, rather than getting frustrated with philosophical questions that spawned precisely from their misleading explanations.

No, I would not. They have nothing to do with each other. Local velocity means something is moving. With cosmic recession, nothing is moving, things are just getting farther apart.

"Things are just getting farther apart", which means they have a relative velocity. Unless you are treating space as an entity that expands between galaxies while galaxies remain at rest relative to that space, which is the fallacy of reification.

General relativity really mucks things up. Locally, General Relativity works exactly like special relativity (so that velocity relationships for objects passing one another are the same). But for far-away objects, relative velocity becomes poorly-defined. Because space-time is curved in General Relativity, you can't directly compare the velocities of two objects.

On the scale of the observable universe space-time is flat, why can't you compare their velocities then?

"Things are just getting farther apart", which means they have a relative velocity. Unless you are treating space as an entity that expands between galaxies while galaxies remain at rest relative to that space, which is the fallacy of reification.

If I pass you in a car, we are moving relative to each other. There is a frame of reference in which I am stationary and you are moving relative to me and there is a reference frame in which you are stationary and I am moving relative to you. If I toss an apple out of the car window as I pass you, and it hits you it is equally valid to say that the apple moved towards the stationary you or you moved towards the stationary apple. Either way, there is momentum that comes into play, as you will well notice when you and the apple meet up. There is NO momentum between galaxies that are receding from each other because they are not moving relative to each other in the way that you and I were moving relative to each other. They are just getting farther apart.

If I pass you in a car, we are moving relative to each other. There is a frame of reference in which I am stationary and you are moving relative to me and there is a reference frame in which you are stationary and I am moving relative to you. If I toss an apple out of the car window as I pass you, and it hits you it is equally valid to say that the apple moved towards the stationary you or you moved towards the stationary apple. Either way, there is momentum that comes into play, as you will well notice when you and the apple meet up. There is NO momentum between galaxies that are receding from each other because they are not moving relative to each other in the way that you and I were moving relative to each other. They are just getting farther apart.

If you never passed me but were always moving away from me you couldn't do that experiment with the apple. With your interpretation it seems you would say there is no momentum involved between two objects that never meet, and so with your interpretation that I couldn't measure the velocity of your car if you didn't pass me but were moving away from me at the beginning. That all I could say is that we are getting farther apart. Yet I'm sure you will agree I can measure the velocity of your car even if we never meet. What's the difference between the two situations then?

I would say that there is a practical difference, not a conceptual one. Practical in that I can measure your velocity almost instantaneously by exchanging light signals, while in the case of distant galaxies one would have to wait billions of years for the light to come back. But still if the distant galaxy is assumed to have a constant relative velocity throughout, its velocity could be measured in principle.

Regarding galaxies supposedly receding faster than light, do we really have any experimental evidence that they do so, besides assuming that Hubble's law applies on cosmological scales?

The problem is it stops being just "philosophy" when statements such as "galaxies move away from each other because space expands" or "light is redshifted because of the stretching of space" are given unscrupulously everywhere. If space is a mathematical framework then it is false to state that the universe works the way it does because of how this mathematical framework behaves, this is the fallacy of reification. They get frustrated with philosophical questions, but they wouldn't have to if they used the proper terminology in the first place.

There's no question that a lot of physicists use sloppy terminology. I've been guilty of it myself. But when it comes down to evaluating ideas, this isn't a problem at all. All physical theories are approximations to the real behavior, and the primary job of physics is to find out where those approximations break down.

With regard to the expansion, for instance, the mathematical framework used (the FLRW metric) assumes that matter is perfectly evenly-distributed throughout space. This is clearly not completely accurate: the very fact that the solar system exists proves it to be false. But it turns out to work very well for describing the universe on large scales. There was also a flurry of work in the early 2000's where they tried to make sure that this approximation did work on large scales as well as was generally believed: some thought that the fact that the universe wasn't uniform might potentially lead to the illusion of an accelerated expansion (this turned out not to be the case).

Why do galaxies move away from each other at a velocity proportional to the distance between them? If you consider space to be a mathematical concept and say because space expands or stretches between them, it's wrong. If you consider space to be an entity, then it is still wrong because this entity hasn't been detected. The valid answer to this question would be "we don't know", or if the big bang is assumed "because that's what they have been doing since the big bang", which then begs the question why that is the case. Explaining why in term of a mathematical concept gives the illusion of an explanation, at best it is an analogy.

Ultimately the fact that the expansion is uniform comes down to initial conditions. This isn't a completely worthless statement: it narrows the scope of the investigation into the causes. What we do know of the expansion is that our universe is pretty accurately described by the cosmological principle. And a universe that obeys the cosmological principle has recession velocity proportional to distance.

So the question then becomes, how was the universe started in such a way that the cosmological principle applies? For that, we don't yet know. The answer may have something to do with inflation, but that appears to just make it so that the cosmological principle needs to be assumed over a smaller region.

In the end it is the non-physicists eager to understand who get frustrated with the misleading explanations given by the physicists. The honest attitude from the physicists then would be to clarify the misconceptions they are responsible for spreading in the first place, rather than getting frustrated with philosophical questions that spawned precisely from their misleading explanations.

If you really want clarity, you have to study the math. It is fundamentally impossible to accurately translate physical theories into words alone. The full description only exists in the mathematical structures that make up the theories.

The above discussion is EXACTLY my point. A recession velocity is not the same as newtonian velocity.

What the general non scientific public who speak english and do not understand the maths understand when physicists talk about a recession velocity. is something receding (moving) with newtonian velocity. Which is incorrect right?

Who pays the majority of physicists salaries and for physics experiments. The general public.

Therefore physicists owe it to the general public to explain what they mean. Asking the general public to learn the maths to understand, is less good a suggestion than asking physicists to be more precise in the language they use when explaining

If it is obvious that commonly used terminology leads to misunderstanding, I would hope physicists would actually think about this. I certainly have.. I suggested a possible solution by suggesting a different terminology to underscore and make obvious to the general public that recession velocity is a different concept to newtonian velocity. Of course physicists know what they mean, and physicists therefore don't need different terminology, but my point is that if you want to get non physicists on the "same page", rather than describing something that immediately conjures up the false idea, and then trying to explain it later. It would be better to describe the different concept at the beginning.

Again, I see physicists have no need when communicating between themselves of new terminology, but when communicating with the general public, I see a need for improvement. Thats all

What we do know of the expansion is that our universe is pretty accurately described by the cosmological principle. And a universe that obeys the cosmological principle has recession velocity proportional to distance.

Wouldn't a universe where recession velocities are the same function of distance for each observer (i.e. not necessarily proportional) obey the cosmological principle too?

If you really want clarity, you have to study the math. It is fundamentally impossible to accurately translate physical theories into words alone. The full description only exists in the mathematical structures that make up the theories.

I wouldn't say fundamentally impossible, I'm sure it could be done but it would take much longer and in the end the explanation using words alone wouldn't be any clearer. But I meant clarifying the misconceptions that have been widespread, such that stating the expansion of space is the cause of galaxies moving away from each other. The vast majority of the public who has heard this explanation believes space to be a real entity that is actually being created or stretched between galaxies, and believe that this is why galaxies move away from each other. What science communicators could do to clarify this is to say that as far as we know space is not an entity, not a real thing, and so that there is no causal link between what we refer to as "space expanding" and galaxies moving away from each other, the former is not a cause of the latter, it is a description to aid visualizing the motion of these galaxies.

Imagine you could attach a long rope to a faraway galaxy with recession velocity. The other end is in your lab on earth.

Will the end of the rope in your lab move?

Lets go a step further. We wrap the end of the rope around a flywheel attached to an electric generator.

Will the flywheel turn and can the generator produce electricity.

The distance between the galaxies increases so indeed if it was attached to the faraway one it would move away from ours and you could generate electricity from that motion, while slowing down the faraway galaxy a tiny bit. That's the way I see it.

Wouldn't a universe where recession velocities are the same function of distance for each observer (i.e. not necessarily proportional) obey the cosmological principle too?

The only possible velocity function that is the same for multiple observers at different locations is one that is proportional to distance. To see this, imagine a single line of galaxies:
x--x--x--x

To make this simple, the nearest-neighbor distance is the same for all of them. So if there is some expansion, some time later it might look like this:
x---x---x---x

It should be relatively easy to see that the recession velocity is proportional to distance. But if the velocity was some other function of distance, at a later time the system might look like this instead:

x--x---x----x

...a situation which is decidedly not homogeneous any longer, and therefore not following the cosmological principle. Also note that the relative velocity of the nearby galaxies according to the observer on the far left would be very different from that seen by the observer on the far right.

What science communicators could do to clarify this is to say that as far as we know space is not an entity, not a real thing, and so that there is no causal link between what we refer to as "space expanding" and galaxies moving away from each other, the former is not a cause of the latter, it is a description to aid visualizing the motion of these galaxies.

I don't think this is a good example. There is really no way to say whether or not space is an "entity" because "entity" isn't well-defined. Space-time does, after all, carry momentum and can transfer energy from one system to another (through gravity waves). Furthermore, there are a number of metrics that we can write down which have no matter but nevertheless have thermodynamic properties. There's every reason to believe space-time just as much a physical thing as the electromagnetic field.

We don't yet know the full picture of what gravity is because we don't have an understanding of quantum gravity.